Foamed fluids are used in many applications, for example as fracturing fluids in the oil field. Although such fluids are commonly called foams, when the second phase is CO2 the fluids are more like an emulsion of water and supercritical CO2 under most application conditions. Furthermore, when the fraction of the non-aqueous phase (called the “foam quality” and sometimes abbreviated here as FQ) is less than about 54%, such fluids are commonly called “energized”. When we say “foamed” we include any quality, including an emulsion; when we say “energized” we mean a quality of less than about 54%. CO2 “foamed” fracturing fluids are often preferred over N2 (or other gas) foamed fracturing fluids under certain circumstances. For instance, CO2 provides additional cleanup energy (relative to N2) when it is gasified as the pressure drops after a treatment is completed. Also, for deeper and hotter wells, CO2 foamed fluids require less horsepower due to their higher density than N2 foamed fluids of equal foam quality. Therefore N2 fluids are rarely pumped over about 180° F. (about 82° C.), and CO2 fluids are routinely used up to about 240° F. (about 116° C.) and higher.
It is a desirable feature to have a fracturing fluid compatible with CO2. Surfactant-based fluids (“VES” or “viscoelastic surfactant” fluids) provide many benefits over conventional polymer fluids when used in fracturing (and for other uses), but most VES fluids are not very compatible with CO2. Not to be limited by theory, but it is believed that this is because the solvency of supercritical CO2 disrupts the micellar structure (for example but not limited to a worm-like micelle structure) that is essential for fluid viscosity. It would be advantageous to make VES/CO2 foamed fluids stable for longer times and/or at higher temperatures.